Ultralight cellulose nanofiber based foams: the effects of material formulation and nanofiber type on mechanical properties and thermal insulation

There is a great demand for sustainable alternatives to petroleum-based foams. Here, we report ultralight, thermally insulating foams, with good mechanical performance made with cellulose nanofibers (CNFs). The CNFs were first suspended in water and crosslinked with polyvinyl alcohol (PVA) followed by freeze drying, to fabricate foam panels. The CNF content in suspension, CNF/PVA ratio, and CNF type were investigated as the main factors controlling the microstructure and properties. The CNF content and CNF/PVA ratio exhibited substantial impacts on the foams’ cellular morphology and mechanical behavior. More uniform cellular structures with smaller pore sizes were obtained at higher CNF contents and higher PVA to CNF ratios. By increasing CNF content from 2 to 6 wt.%, the compressive modulus, strength, and flexural modulus all increased up to 26, 9, and 7 folds, respectively, with only about 3% loss in the porosity. The thermal conductivity was consistent in the range of 0.03–0.05 W/mK and changed only slightly with the CNF content and CNF/PVA ratio. Moreover, the results revealed that the additional refining of CNF did not positively affect the properties. This implies less refined and more cost-effective CNF feedstock can be utilized for this foaming method, as an essential step towards scale up.